This invention relates to vortex flow sensors for measuring the flow velocity and/or the volumetric flow rate of a fluid flowing in a measuring tube.
Conventional vortex flow sensors have a bluff body which is disposed along a diameter of the measuring tube and fixed in the wall of this tube.
During operation of such a vortex flow sensor, as is well known, a Karman vortex street is formed downstream of the bluff body. Its pressure fluctuations are converted by a sensing element into an electric signal whose frequency is proportional to the fluid velocity, from which the volumetric flow rate can be calculated.
The vortex sensing elements used so far are, on the one hand, devices which extend into the fluid and on which the pressure fluctuations act directly; such devices are, for example, pressure sensors, particularly capacitive ones, which are mounted in the bluff body or which are mounted in or inserted through the wall of the measuring tube down-stream of the bluff body.
On the other hand, the pressure fluctuations have been measured by means of an ultrasonic arrangement whose transmitter and receiver are mounted outside the measuring tube diametrically opposite each other. The transmitter sends ultrasonic signals through the wall of the measuring tube, the fluid, and the opposite wall which are modulated by the pressure fluctuations and registered by the receiver.
Each of these two types of sensing elements has inherent disadvantages which preclude vortex flow sensors fitted with such sensing elements from being used with any fluid. Ultrasonic sensors can be used only up to fluid temperatures of approximately 250xc2x0 C., and capacitive pressure sensors only up to fluid temperatures of approximately 400xc2x0 C.
Furthermore, vortex flow sensors have been described in which a light or laser beam is sent through the fluid and the intensity modulation of the beam by the vortices serves to determine the vortex frequency; see, for example, U.S. Pat. No. 4,519,259 or GB-A 2,084,720. Such optical vortex flow sensors also withstand temperatures higher than 400xc2x0 C.
It is an object of the invention to provide an optical sensor system which is also suitable for use at temperatures higher than 400xc2x0 C., does not come into contact with the fluid, and requires less space than conventional optical sensor systems.
To attain this object, a first variant of the invention provides a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluid, said vortex sensor comprising:
a measuring tube
through which the fluid flows in a first direction and
which has a wall in which a first window and a second window of optical, schlieren-free, high-temperature glass are set fluid-tight and pressure-tight at points lying opposite each other along a first diameter of the measuring tube;
a bluff body disposed along a second diameter of the measuring tube and fixed in the measuring tube for generating Kxc3xa1rmxc3xa1n vortices in the fluid, whose frequency is proportional to the flow velocity,
said second diameter lying upstream of, and being essentially perpendicular to, the first diameter; and
a laser differential interferometer having
a transmitting unit mounted outside the measuring tube in front of the first window and fixed to the first window and/or the measuring tube, said transmitting unit comprising the following optical components, which follow one another in the direction to the first window:
a laser,
a lens system,
a first polarization filter,
a first Wollaston prism, and
a first lens, and
a receiving unit mounted outside the measuring tube in front of the second window and fixed to the second window and/or the measuring tube, said receiving unit comprising the following optical components, which follow one another in a direction away from the second window:
a second lens,
a second Wollaston prism,
a second polarization filter, and
a PIN diode.
To attain the above object, a second variant of the invention provides a vortex flow sensor for measuring the flow velocity and/or the volumetric flow rate of a fluid, said vortex sensor comprising:
a measuring tube
through which the fluid flows in a first direction and
which has a wall in which a first window and a second window of optical, schlieren-free, high-temperature glass are set fluid-tight and pressure-tight at points lying opposite each other along a first diameter of the measuring tube;
a bluff body disposed along a second diameter of the measuring tube and fixed in the measuring tube for generating Kxc3xa1rmxc3xa1n vortices in the fluid, whose frequency is proportional to the flow velocity,
said second diameter lying upstream of, and being essentially perpendicular to, the first diameter; and
a laser differential interferometer having
a transmitting unit mounted outside the measuring tube in front of the first window and fixed to the first window and/or the measuring tube, said transmitting unit comprising the following optical components, which follow one another in the direction to the first window:
a laser emitting polarized light,
a lens system,
a first Wollaston prism, and
a first lens, and
a receiving unit mounted outside the measuring tube in front of the second window and fixed to the second window and/or the measuring tube, said receiving unit comprising the following optical components, which follow one another in a direction away from the second window:
a second lens,
a second Wollaston prism,
a polarization filter, and
a PIN diode.
In a preferred embodiment of the first or second variant of the invention, the first and second lenses are achromats.
One advantage of the invention is that even fluids having a temperature of, e.g., 600xc2x0 C. can be measured on the vortex principle.
Another advantage results from the fact that the optical components can be fabricated using microsystem technology, so that the optical sensor system takes up less space than conventional sensor systems.